Pneumocystis carinii is an important cause of morbidity and mortality in immunocompromised patients, with the highest attack rate occurring in patients with the acquired immunodeficiency syndrome (AIDS). Current understanding of this organism is still rudimentary; consequently, the epidemiology and pathogenesis of disease produce by P. carinii, as well as the role of host immunity to specific antigens, the understanding of the antigenic characteristics of P. carinii in order to develop improved modes of prevention and/or therapy of P. defining the antigens of P. carinii important in acquired resistance to infection, and by developing an understanding of the polymorphism of P. carinii antigens, it will be possible to identify vaccine candidates for use in passive or active immunization. Furthermore, characterizing the host species-specific polymorphisms of P. carinii, as a means of defining P. carinii serotypes, will facilitate our understanding of the epidemiology of P. carinii pneumonitis. To test this hypothesis the following specific aims will be completed: 1) identify specific antigens of P. carinii by reactivity with monoclonal antibodies (Ma) to P. carinii from various hosts; 2) characterize the P. carinii antigens identified by the Ma produced in aim 1; 3) clone, characterize and express the gene encoding the 37 kd antigen of ferret P. carinii which has an epitope common to P. carinii from other hosts; and 4) examine the potential role of P. carinii ga in active and passive immunotherapy of P. carinii pneumonitis.